Kruge had just murdered Kirk's son after torturing him for information hours after obliterating the entire crew of the USS Grissom and attacking the Enterprise. Still, after all that, when Kruge was in danger of falling, Kirk offered his hand. Kruge's response was to pull Kirk down into the lava with him. To save himself and Spock, Kirk did the only thing he could: he kicked his adversary over the side. At that point, Kruge had done one thing after another and another, so Kirk's exasperation added the "I have had enough of you" to it.

Self defense / justifiable homicide / justice. Executing Khan and his followers after having them in custody would be brutal, heartless murder. Sorry, not in support of that.